ABSTRACT
Stress analysis using numerical methods being increasingly accepted for design of underground excavations due to their ability to consider most of the complexities and their reliability to produce realistic information. The paper presents a design approach based on the stress analysis of the twin tube 18m wide 9.5m high tunnels with rock covers ranging from 10m to 135m. Two-dimensional discrete element modeling using UDEC version 3.0 is used for stress analysis. The support system has been designed based on the deformations in the rock mass and the slip and separation of the discontinuities around the excavation. The support system consisting of 100mm to 250mm shotcrete with 4m to 6m long bolts at 1.5m spacing is recommended based on the magnitude of joint separation, slip and deformation in the rock mass around the excavation. Then support system is incorporated in the model to evaluate its efficacy. The rock bolts were modelled as elements with axial and shear stiffness and the shotcrete as beam element with finite interface strength. The adequacy of the thickness of shotcrete and length of bolts are verified based on the loads induced in them.
